When “once in a while” answer is insufficient and you need to know exactly how often a particular event occurs on the system, there are a few easy things you can try from command line.

In the examples below, we’re looking at the vsftpd.log to determine the number of successful logins on the hourly, daily, and monthly basis. First, hourly distribution of logins:

[root@vsftp01~]# p="\] OK LOGIN" ; log="/var/log/vsftpd.log" ; zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | cut -d' ' -f2-4 | cut -d: -f1 | sort -k1M -k2n -k3n -u | while read line ; do c=$(zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | grep -c "${line}") ; d=$(date --date="${line}" +'%Y-%m-%d %H %p') ; echo -e "${d}:\t${c}" ; done

2019-03-22 03 AM:       5
2019-03-22 04 AM:       7
2019-03-22 05 AM:       9
2019-03-22 06 AM:       12
2019-03-22 07 AM:       7
2019-03-22 08 AM:       19
2019-03-22 09 AM:       20
2019-03-22 10 AM:       21
2019-03-22 11 AM:       19
2019-03-22 12 PM:       19
2019-03-22 13 PM:       19
2019-03-22 14 PM:       19
2019-03-22 15 PM:       19

Now daily:
[root@vsftp01 ~]# p="\] OK LOGIN" ; log="/var/log/vsftpd.log" ; zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | cut -d' ' -f2-3 | sort -k1M -k2n -u | while read line ; do c=$(zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | grep -c "${line}") ; d=$(date --date="${line}" +'%Y-%m-%d') ; echo -e "${d}:\t${c}" ; done

2019-03-22:     304
2019-03-23:     3149
2019-03-24:     9188
2019-03-25:     3209
2019-03-26:     1248
2019-03-27:     1883
2019-03-28:     339
2019-03-29:     338
2019-03-30:     1421

Here’s a fancier version that’s useful for identifying daily patterns. As you can see, I had to resort to Bash shell’s more sophisticated counterpart – the Korn shell – still unsurpassed in performance and functionality since the last version came out in 1993:
[root@vsftp01 ~]# /bin/ksh
# p="\] OK LOGIN" ; log="/var/log/vsftpd.log" ; i=0 ; zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | cut -d' ' -f2-3 | sort -k1M -k2n -u | while read line ; do c=$(zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | grep -c "${line}") ; d=$(date --date="${line}" +'%Y-%m-%d %a') ; a1[$i]="${d}" ; a2[$i]="${c}" ; (( i = i + 1 )) ; done ; w=80 ; max=$(printf '%s\n' "${a2[@]}" | sort -nr | head -n1) ; x=$(echo "scale=4;${w}/${max}"|bc -l) ; for i in $(seq 0 `echo "scale=0;${#a1[*]}-1"|bc -l`) ; do echo -ne "${a1[$i]}\t${a2[$i]}\t" ; for j in $(seq 0 `echo "scale=0;$(echo ${a2[$i]})*${x}/1"|bc -l`) ; do echo -n "." ; done ; echo "" ; done
2019-03-22 Sun  304     ...
2019-03-23 Mon  3149    ............................
2019-03-24 Tue  9188    ................................................................................
2019-03-25 Wed  3209    ............................
2019-03-26 Thu  1248    ...........
2019-03-27 Fri  1883    .................
2019-03-28 Sat  339     ...
2019-03-29 Sun  338     ...
2019-03-30 Mon  1421    .............
2019-03-31 Tue  1131    ..........
2019-04-01 Wed  1211    ...........
2019-04-02 Thu  260     ...

And monthly:
[root@vsftp01 ~]# p="\] OK LOGIN" ; log="/var/log/vsftpd.log" ; zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | cut -d' ' -f2 | sort -k1M -u | while read line ; do c=$(zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | grep -c "${line}") ; d=$(date --date="${line} 1" +'%Y-%b') ; echo -e "${d}:\t${c}" ; done
2019-Mar:       22210
2019-Apr:       1420

If you feel like some serious scripting, here’s an example that will show you the daily distribution of vsftpd successful and failed logins, as well as successful and failed downloads and uploads. Metrics like this can be useful in catching problems early on, before too many users get upset:
[root@vsftp01 ~]# log="/var/log/vsftpd.log" ; k=0 ; array=( "OK LOGIN" "FAIL LOGIN" "OK DOWNLOAD" "FAIL DOWNLOAD" "OK UPLOAD" "FAIL UPLOAD" ) ; for d in {7..0}; do date -d "`date +'%Y-%m-%d'` - $d days" +'%b %-d'; done | while read d ; do if [ ${k} -eq 0 ] ; then echo -ne "PERIOD," ; printf "%s," "${array[@]}" | sed 's/,$//g' ; echo "" ; k=1 ; fi; j=1 ; for i in "${array[@]}" ; do p="\] ${i}"; eval "$(echo c${j})"=$(zgrep -E "${p}" "${log}"* | tr -s ' ' | cut -d: -f2- | grep -c "${d} ") ; (( j = j + 1 )) ; done; echo -ne "`date -d "${d}" +'%Y-%m-%d'`," ; for j in $(seq 1 `echo "${#array[@]}"`) ; do eval echo -ne $(echo $`eval echo "c${j},"`); done | sed 's/,$//g' ; echo "" ; done
PERIOD,OK LOGIN,FAIL LOGIN,OK DOWNLOAD,FAIL DOWNLOAD,OK UPLOAD,FAIL UPLOAD
2019-03-26,1248,72,3635,2,512,0
2019-03-27,1883,70,2524,2,410,0
2019-03-28,339,2,46,2,10,0
2019-03-29,338,4,11,3,13,0
2019-03-30,1421,49,950,3,549,0
2019-03-31,1131,71,1684,2,401,0
2019-04-01,0,0,0,0,0,0
2019-04-02,0,0,0,0,0,0

This approach can be relatively easily adopted to most standard system and application logs and used to track distribution over time of most uniquely-identifiable events. As long as you can grep for something predictable and there is a time stamp, you can use this method.